With the advent of Vista and Word 2007, the issues suddenly got
considerably more difficult. So let's start with the easiest solution.

Upgrade to MathType 6

This solution works very easily - simply install MathType 6 after
installing Word 2007. You will then find a MathType Tab on the Quick Access
Toolbar when you open Word 2007. This solution is elegantly simple and logical,
but it does cost you the price of an upgrade. Of course, you can look for
MathType 6 Lite - that would serve your needs again and would have no problem
installing itself harmoniously with Word 2007. The address for the Free 30-day
Evaluation is

In the past, once the 30-day evaluation period was over, the software
reverted to a version that was still very serviceable for most of our needs. I
believe that is also true of the new version.

When you have installed MathType 6 with Word 2007, you will find a
MathType Tab on teh Quick Access Toolbar, and it will look like this:

Word 2007 comes with its own new Editor for math expressions. That
in large part would serve most ot the needs of the users. But if you are
accustomed to MathType 5 and do not want to upgrade to MathType 6, and you do
not want to use the built-in editor, you will have to deal with the fact that
Word 2007 has some road blocks in place. You can of course try to get around
those road blocks. So before I go on, let me repeat that upgrading is most
likely the easiest and best way to get the job done. Either upgrade or download
MathType 6 "30-day Evaluation" and let it lapse into a lighter be still
serviceable version.

For details on this there is probably no better place to go but the
company's page that gives the instructions. The deal with issues like Vista/XP
as well. That web page is
http://www.dessci.com/en/support/mathtype/tsn/tsn123.htm When you are
done with their instructions, you will find MAthType 5 in teh Add-Ins Command
Tab:

If, after reading the instructions (as I suspect) you decide that you
would rather upgrade, then try the 30-day evaluation copy first and get the full
upgrade later if you decide it is worth doing so. Again, to download the free
copy for Windows operatins system, go to
http://www.dessci.com/en/products/mathtype/trial.asp?src=linkBox.